The present disclosure generally relates to methods and devices for forming images on flexible substrates. In this regard, this disclosure relates to forming or printing images on flexible surfaces such as those used in packaging and other applications, through the use of curable phase change inks.
Conventional ink jet printing systems, such as drop-on-demand or continuous systems, are known in the art. In drop-on-demand systems, a droplet is expelled from the orifice of a printhead directly to a position on a recording medium, such as paper. A droplet is not formed or expelled unless it is to be placed on the recording medium.
An example of such a drop-on-demand system is known as thermal ink jet (TIJ), or bubble jet. It produces high velocity droplets by passing a current pulse through a resistive layer within the ink passageway near the nozzle, causing the ink vehicle in the immediate vicinity to vaporize and expel droplets of ink from the nozzle.
Another example of a drop-on-demand system is piezoelectric ink jet (PIJ). In such a system, the printhead has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end. The piezoelectric transducer produces pressure pulses which expel drops of ink from the nozzle.
A further example of a drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the acoustic pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the acoustic pressure it exerts for a given amount of input power.
Printing on flexible packaging substrates may be useful for various applications. The term “flexible” is intended to refer to thin film or foil materials which are generally supplied in a roll format, printed on, and then rolled up again after printing. Exemplary flexible packaging substrates include plastics, metal foils, laminates thereof, and laminates with paper. Flexible packaging substrates can be used, for example, to package food, pharmaceuticals, cosmetics, or tobacco.
Currently, digital printing on flexible packaging substrates is often done with liquid toners or ultraviolet-curable inks. Liquid toners suffer from image robustness problems and are not durable. For example, they have poor scratch, rub, and smear resistance. Conventional UV-curable inks have low viscosity. As a result, they tend to flow laterally, especially on non-porous flexible packaging substrates, resulting in irregular dot growth. This lateral spreading can be decreased by pinning, which is a partial cure of the UV ink to increase its viscosity. In addition to prevention of lateral spreading, pinning is often done after printing each color in color images, so that the next color will impinge onto a stable ink surface and not mix and/or flow with the ink printed previously. The UV lamps required for pinning represent additional equipment cost which increases the cost of the printed flexible substrate. The pinning lamps also require additional power which also increases the cost of the printed substrate. Moreover the space required for the pinning lamps lengthens the print zone or waterfront required, which results in a larger printer needing more floor space. The ability to fix the entire image in a final cure step downstream from the print zone is also beneficial since it reduces the chance of stray UV light from a neighboring pinning lamp entering the printhead and prematurely curing the ink, resulting in printhead fouling. Additionally, pinning is a complex process with a narrow operating window requiring sufficient cure to render the ink immobile, but ink that is overcured in the pinning step offers poor adhesion for the next color. This complexity is compounded as the first colors deposited are exposed to additional pinning or curing cycles as subsequent colors are deposited and pinned.
Additional means that allow for forming images or printing on flexible substrates are still desired.